In forging of hardy workable metal materials such as titanium alloy and Ni-based alloy, as a method for hot precise die forging, an isothermal forging method of performing forging by holding die temperature at the same level as the heating temperature of a forging material and controlling distortion rate within a fixed range is increasingly applied. Further, a hot die forging method of performing forging by controlling the distortion rate while bringing the die temperature closer to the heating temperature of the forging material is also increasingly applied.
As a die structure used for the above-mentioned isothermal forging or hot die forging, a hot forging press apparatus 71 as shown in FIG. 4 is given as an example. In the hot forging press apparatus 71, dies 73, 74 heated by a heat source 72, for example, an induction heater or the like are mounted on die plates (die holding means) 77, 78 fixed to base plates 75, 76 of a press apparatus body. Heat insulating structural members 79, 80 are disposed respectively between the dies 73, 74 and the die plates 77, 78 to prevent the transfer of heat from the dies 73, 74 heated by the heat source 72 toward the die plates 77, 78 and the press apparatus body. In the above-mentioned hot precise die forging of hardly workable materials, the upper and lower dies are heated to a temperature close to the heating temperature of the forging material. Therefore, from the viewpoint of formability and operation performance, the die temperature must be accurately and efficiently controlled. The accurate control of the die temperature requires consideration of heating of the die device including not only the dies 73, 74 but also the die plates 77, 78.
On the other hand, means for efficiently heating a forging die is disclosed, for example, in Patent Document 1 . In a heating jig used in the heating method of Patent Document 1, a heater is provided in the middle between a first die (upper die) and a second die (lower die) disposed oppositely to each other. Further, one surface of the first die (upper die) and the second die (lower die) is formed in a shape taken along a forging surface of the first die, and the other surface is formed in a shape taken along a forging surface of the second die.